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Closures

クロージャ

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One of the things that makes Groovy different than most compiled languages is that you can create functions that are first class objects. That is you can define a chunk of code and then pass it around as if it were a string or an integer. Check out the following code:

The curly braces around the expression "it * it" tells the Groovy compiler to treat this expression as code. In the software world, this is called a "closure". In this case, the designator "it" refers to whatever value is given to the function. Then this compiled function is assigned to the variable "square" much like those above. So now we can do something like this:

This is not very interesting until we find that we can pass this function "square" around as a value. There are some built in functions that take a function like this as an argument. One example is the "collect" method on arrays. Try this:

This expression says, create an array with the values 1,2,3 and 4, then call the "collect" method, passing in the closure we defined above. The collect method runs through each item in the array, calls the closure on the item, then puts the result in a new array, resulting in:

By default closures take 1 parameter called "it", you can also create closures with named parameters. For example the method Map.each() can take a closure with two variables, to which it binds the key and associated value:

Here are a few more closure examples. This first one shows a couple of things. First, the closure is interacting with a variable outside itself. That is, the closure's purpose is to put together the parts of a stock order held in the array orderParts, by adding (appending) it to the variable fullString. The variable fullString is not in the closure. The second thing that is different about this example is that the closure is "anonymous", meaning that it is not given a name, and is defined in the place where the each method is called.

Dealing with Files

Reading data from files is relatively simple. First create a text file, and call it myfile.txt. It doesn't matter what's in it, just type some random text into it and save it on your C: drive in the \temp directory. Then type the following code in the groovyConsole:

This should print out every line in the file prefixed with "File line: ". The first two lines of the code simply declare variables to specify where the file is located. The variable names don't have any special significance, and as you can see, all we do is combine them when we use them. Note that because the backslash character has special meaning in groovy, you have to use two of them to tell it that you '''really''' mean a backslash.

The next line that starts "myFile =" creates a new File object. An object is simply a collection of related methods and data. For example, a file object might have data describing its location, in this case "C:\temp\myfile.txt", and maybe a method to delete the file if it exists. In this case the only method we are going to use is the eachLine method, which we call in the last line of code. The line before that is a simple closure definition, that you have seen several times by this point.

Dealing with strings

Strings in Groovy have all the same functionality of Java strings. That is, a Groovy string is just a Java string with a few extra things added to it. Because of that, we can refer to the Java documentation for the String class to find out some of the interesting things we can do with it. For example, look in the section entitled '''Method Summary''' at the description for the '''split''' method. This method does something very useful, which is to split a string based on a regular expression. We will talk more about regular expressions later, but for now the only thing you have to know is that the simplest regular expression is a single character. So let's say that we want to split up the components of the date "2005-07-04", so that we can add one to the year to get the date of next fourth of July. We might:

This code brings together a bunch of things we have talked about before. There are two new things, first is the use of the split method on a String. Second is the call of toInteger() on a String. This call to toInteger simply tells Groovy that you want to treat that data as a number rather than a String. See what happens if you run the same code without ".toInteger()" at the end of the third line.

Another thing you might notice is that toInteger is not listed in the Java documentation for string. That is because it is one of the extra features that Groovy has added to Strings. You can also take a look at the documentation for the Groovy extensions to Java objects.